1. Field of the Invention
This invention relates to an engine speed calculation apparatus for calculating a speed of an engine.
2. Description of Background Art
Conventionally, in calculation of an engine speed, a time required for rotation of a crankshaft over a predetermined angle is measured, and an engine speed is determined in accordance with the required time. More particularly, a required time corresponding to 360 degrees as a predetermined angle is measured from generation timings of crank pulses obtained from a crank angle sensor, and a reciprocal number to the required time is multiplied, for example, by 60 to calculate the engine speed. The engine speed calculated in this manner is used as a principal engine operation parameter in engine control such as fuel injection control (for example, refer to Japanese Patent Laid-Open No. Sho. 61-277845 and Japanese Patent Laid-Open No. Hei. 9-264241).
However, in a low speed region wherein the engine speed is low, even when the engine speed should be kept constant, the engine speed pulsates when compared with that in a high speed region, therefore, if the engine speed is calculated using a time required for rotation only over an angle equal to that in the high speed region, an engine speed which is influenced by the pulsating rotation is calculated, and successively calculated engine speeds are fluctuated up and down. If a calculated engine speed influenced by pulsation rotation in this manner is used for engine control, then smooth engine control cannot be achieved. This subject is more conspicuous with an engine in which explosion occurs at non-uniform intervals such as a V-type engine.
For example, in the case of an L-type four-cylinder engine in which ordinary uniform interval explosion is performed, explosion of one cylinder is performed every 180 degrees of rotation of the crankshaft, and the explosion is repeated in the order of a first cylinder, a third cylinder, a second cylinder and a fourth cylinder as shown in (a) of FIG. 1. Further, as shown in (b) of FIG. 1, different strokes, that is, an expansion stroke, an exhaust stroke, an intake stroke and a compression stroke are performed in synchronism with one another at intervals of 180 degrees in the crank angle for all cylinders, and further, the strokes of the first to fourth cylinders for 180 degrees in crank angle are different strokes from each other over 720 degrees in crank angle. Consequently, within a period of any interval of 180 degrees, each of an expansion stroke, an exhaust stroke, an intake stroke and a compression stroke is performed by one of the four cylinders, and a same stroke is not performed simultaneously by a plurality of cylinders. In other words, an expansion stroke which accelerates rotation of the crankshaft because the piston is pushed by an explosion pressure and an expansion stroke which decelerates the rotation of the crankshaft because the intake mixture is compressed are performed simultaneously at an interval of 180 degrees and the balance is kept comparatively well, and thus the rotation of the crankshaft is averaged. Consequently, calculation of the engine speed is less liable to be influenced by pulsating rotation, except a variation among the cylinders.
On the other hand, in the case of a V-type four-cylinder engine in which non-uniform interval explosion occurs, although explosion is repeated in order of a first cylinder, a third cylinder, a second cylinder and a fourth cylinder as shown in (a) of FIG. 2, if explosion of the first cylinder occurs at 0 degree of the crank angle, then the crank angle from the explosion of the first cylinder to the explosion of the third cylinder is 180 degrees; the crank angle from the explosion of the third cylinder to the explosion of the second cylinder is 270 degrees; the crank angle from the explosion of the second cylinder to the explosion of the fourth cylinder is 180 degrees; and the crank angle from the explosion of the fourth cylinder to the explosion of the first cylinder is 90 degrees. This is because, although the different strokes, that is, the expansion stroke, exhaust stroke, intake stroke and compression stroke, are performed at intervals of 180 degrees in the crank angle in each of the cylinders as shown in (b) of FIG. 2, the first and third cylinders and the second and fourth cylinders are displaced by 90 degrees in variation positions between the strokes. Therefore, in a period of one cycle within which the crankshaft rotates twice from 0 degree to 720 degrees in the crank angle, the number of times for an explosion is greater in a period of the latter half rotation from 360 degrees to 720 degrees of the crank angle than that of the former half rotation from 0 degree to 360 degrees of the crank angle, and there is a period of 270 degrees present from the point in time of the explosion of the third cylinder to the explosion of the second cylinder. Therefore, immediately prior to the explosion of the second cylinder, the rotation of the crankshaft is in a deceleration tendency. On the contrary, since the period from the point in time of the explosion of the fourth cylinder to the explosion of the first cylinder is as short as 90 degrees, before and after the explosion of the first cylinder, the rotation of the crankshaft is in an acceleration tendency. Further, in the period of 0 to 90 degrees (the period indicated by reference symbol A) within the one cycle, expansion strokes occur in an overlapping relationship with the first cylinder and the fourth cylinder to each other and the acceleration of rotation of the crankshaft is in an increasing tendency. Further, in the period from 360 degrees to 450 degrees (the period indicated by reference symbol B), the expansion stroke occurs with none of the cylinders and the compression stroke occurs only with the second stroke, and consequently, the rotation of the crankshaft is in a deceleration tendency. As a result, in the case of an engine in which non-uniform interval explosion occurs, such as a V-shaped four-cylinder engine as described above, pulsation of the engine rotation is different between a period of a first half rotation from 0 degrees to 360 degrees of the crank angle and another period of a second half rotation from 360 degrees to 720 degrees of the crank angle, and particularly in a low speed region, the calculation of the engine speed is liable to be influenced by the pulsation of rotation.
Consequently, if it is tried to perform calculation processing while calculation of the engine speed is not influenced by pulsation of rotation in a low speed region, then there is the possibility that the calculation processing may be complicated.